As is known in the art, when it is necessary to coordinate interaction between two machines, issues related to control of the machines can arise.
Mechanical interaction with objects is arguably one of the fundamentally important robot behaviors. Many current robot applications require it. For example, mechanical interaction is essential for manipulation and the core task of assembly systems. Future robot applications such as versatile use of tools or close cooperation with humans may be enabled by improved control of mechanical interaction.
Interaction with the environment may serve sensory or motor functions (or both) and the most appropriate mechanical interaction is different for sensory or motor tasks. Mechanical interaction dynamics may be characterized by mechanical impedance, which may loosely be considered a dynamic extension of stiffness. Lower mechanical impedance reduces interaction forces due to encountering an unpredicted object, thereby protecting both the robot and any object it manipulates (interaction forces on each being opposite but equal). Using a human analogy, by this reasoning, tactile exploration and manipulation of fragile objects should evoke the use of our lowest-impedance limb segments and while we can (and routinely do) interact with objects using other body parts (the elbow, the knee, the foot, etc.) we naturally tend to use our fingers for gentle, delicate tasks.
Conversely, wielding an object such as a tool often requires it to be stabilized and that requires higher mechanical impedance. This is particularly important if the interaction between the manipulator and the object is destabilizing, as is the case for many common tools. Again using a human analogy, consider, for example, the simple task of pushing on a surface with a rigid stick. If force is exerted on the stick normal to the surface, then the stick is statically unstable; small displacements from the configuration in which stick axis and force vector co-align result in torques that act to drive the stick further from that configuration. Success at this task requires a stabilizing mechanical impedance and because pushing harder exacerbates the problem (the magnitude of the destabilizing torque is proportional to the applied force) the minimum required impedance grows with the force applied. Simple though this task may be, it is an essential element of the function of many tools (e.g., screwdrivers, power drills, etc.) and any manipulator—human or robotic—must provide a stabilizing mechanical impedance to operate them.
In other applications a robot's interactive behavior may be the main objective of control. For example, to use a robot to serve as a force-reflecting haptic display or to deliver physiotherapy requires intimate physical interaction with humans. In these applications the “feel” of the robot becomes an important performance measure, and “feel” is determined by mechanical interaction dynamics. Versatile interaction with objects (whether tools or humans or other robots) therefore requires an ability to modulate and control the dynamics of interaction.
Traditional haptic devices are used as interfaces for computers, surgical simulations, as controls for tele-operating robots, and have recently been used in the first trans-oceanic “touch.” These devices, however, necessarily exchange very little power with the user, and therefore are not useful for other markets. High force haptic devices could be used to create versatile, programmable exercise machines, physically accurate vehicle simulators, rehabilitation and other medical robots and robots that can cooperatively assemble heavy and tight-fitting objects, in addition to higher-force, more realistic versions of the traditional applications. For example, higher force capabilities would provide a larger operational range for a computer interface, permitting a dramatic increase in the capacity to deliver information to and from a computer via force. The difficulty of assuring a stable exchange of power while providing a desired “feel” for systems that exchange significant power across ports of interaction with their environments has limited the use of such devices.
High impedance machines have been adapted to behave as if their actual impedance is lower using force feedback. The main problem with this artificial method is that it introduces instability when coupled to certain environments (contact or coupled instability). It is widely understood that this happens because beyond certain very small force feedback gains, the system becomes non-passive, meaning that it is capable of delivering non-prescribed energy to the interaction port.
Recent work has shown that introducing an appropriately placed and sized dissipative element in parallel with a spring and in series between the manipulator and environment, can drain the energy that force-based control generates and passivate the system.
In view of the above, suffice it to say that a large class of applications, including robots that interact with humans, demands interactive robots with low mechanical impedance. The most direct approach is to design low-impedance hardware and use a simple impedance control algorithm; in fact, this is the recommended approach. However, intrinsically low-impedance hardware can be difficult to create, particularly with complex geometries and large force or power outputs. Most robotic devices have intrinsically high friction and/or inertia and the simple impedance control technique uses the robot actuators only to increase robot impedance. It does nothing to reduce intrinsic robot impedance. Considerable effort has been devoted to designing alternative controllers to reduce the apparent endpoint impedance of interactive robots.